Power devices, structures, components, and methods using lateral drift, fixed net charge, and shield

ABSTRACT

Lateral power devices where immobile electrostatic charge is emplaced in dielectric material adjoining the drift region. A shield gate is interposed between the gate electrode and the drain, to reduce the Miller charge. In some embodiments the gate electrode is a trench gate, and in such cases the shield electrode too is preferably vertically extended.

CROSS-REFERENCE

Priority is claimed from copending U.S. Application 61/225,009 filed on13 Jul. 2009.

Priority is also claimed from commonly owned U.S. application Ser. No.11/971,152 filed 8 Jan. 2008 and now published as US 20080164520, andtherethrough from provisional application 60/879,434 filed 9 Jan. 2007.

Priority is also claimed from commonly owned U.S. application Ser. No.12/432,917, filed 30 Apr. 2009 and now published as US 2010/0025726, andtherethrough from provisional application 61/084,639 filed 30 Jul. 2008.

Priority is also claimed from commonly owned U.S. application Ser. No.12/431,005, filed 28 Apr. 2009 and now published as US 2010/0025763, andtherethrough from provisional applications 61/084,639 and 61/084,639,both filed 30 Jul. 2008.

Each and every one of these priority applications is hereby incorporatedby reference.

BACKGROUND

The present application relates to lateral power semiconductor devices,and more particularly to lateral power semiconductor devices whichincorporate fixed or permanent electrostatic net charge.

Note that the points discussed below may reflect the hindsight gainedfrom the disclosed inventions, and are not necessarily admitted to beprior art.

Power MOSFETs are widely used as switching devices in many electronicapplications. In order to minimize the conduction power loss it isdesirable that power MOSFETs have a low specific on-resistance (R_(SP)or R*A), which is defined as the product of the on-resistance of theMOSFET multiplied by the active die area. In general, the on-resistanceof a power MOSFET is dominated by the channel resistance and the driftregion resistances which include the channel resistance, spreadingresistance and the epitaxial layer resistance.

In the parent applications referenced above, and in many other commonlyowned applications, the present inventors have proposed a variety of newvertical and lateral structures which improve on-resistance and/orbreakdown characteristics by incorporating fixed or permanent charges.FIGS. 1(a)-1(c), 2(a), and 2(b) show lateral device structures disclosedin the various parent applications, but it is important to note thatthose structures are not prior art against the present application,since priority has been claimed back to their disclosures.

SUMMARY

The present application discloses new approaches to lateral andquasi-lateral device structures, in which a shield electrode is addedinto a structure which includes immobile net electrostatic charge.

The disclosed innovations, in various embodiments, provide one or moreof at least the following advantages. However, not all of theseadvantages result from every one of the innovations disclosed, and thislist of advantages does not limit the various claimed inventions.

-   -   Reduced gate-drain capacitance;    -   Reduced Miller charge; and    -   Faster switching.

BRIEF DESCRIPTION OF THE DRAWINGS

The disclosed inventions will be described with reference to theaccompanying drawings, which show important sample embodiments of theinvention and which are incorporated in the specification hereof byreference, wherein:

FIGS. 1(a)-1(c), 2(a), and 2(b) show lateral device structures disclosedin the various parent applications.

FIGS. 3(a)-3(c) together show a first sample embodiment of a new classof innovative structures.

FIG. 4 shows an embodiment using source metal to form the shield layer.

FIGS. 5(a)-5(c) show another sample embodiment.

FIG. 6 shows another alternative embodiment which includes a shallowsurface layer, doped opposite to the body, at the surface of the driftregion.

FIGS. 7(a)-7(e) show other embodiments which include an n-type driftlayer, a P-type surface layer and shallow dielectric trench.

FIGS. 8(a) and 8(b) show other alternative embodiments which includes anN-buried layer within the drift region in FIG. 8(a) and a shallow trenchdielectric in FIG. 8(b).

FIGS. 9(a)-9(c) show different SOI embodiments.

FIG. 10 shows another SOI embodiment using trench gate and shield.

FIGS. 11(a) and 11(b) show other device embodiments, in which thedielectric-filled trenches are laterally tapered.

DETAILED DESCRIPTION OF SAMPLE EMBODIMENTS

The numerous innovative teachings of the present application will bedescribed with particular reference to presently preferred embodiments(by way of example, and not of limitation). The present applicationdescribes several inventions, and none of the statements below should betaken as limiting the claims generally.

In the device shown in FIGS. 1(a)-1(c), permanent positive electrostaticcharges are incorporated in trenches 140 filled with a dielectricmaterial such as silicon dioxide. This can be done, for example, byimplanting Cesium ions into a thin layer of oxide before the trench isfilled. Optionally, permanent charge can also be incorporated in thedevice's surface dielectric layer. The permanent charge shapes theelectric field at reverse bias and results in a higher breakdownvoltage. In the on-state the permanent charge forms an electron induceddrift region in a power MOSFET by forming an inversion layer along theinterface between the oxide and a surrounding p-type layer (e.g. ap-type epitaxial or p-well layer 108, as shown in FIGS. 1(b) and 1(c)).By making use of this new concept, a small cell pitch and high packingdensity can be realized to reduce the device's total on-resistance andspecific on-resistance (R_(SP)).

Thus in normal operation of the structure of FIGS. 1(a)-1(c), asufficiently positive voltage on gate 130 will invert a surface portionof the p-type body 160 to create a channel, and thereby permit electronsto flow from n+ source diffusion 110 through the channel to a portion ofp-type epitaxial layer 108 which has been inverted by the fixed chargeat a dielectric interface. These inverted portions will be referred toas induced drain extensions. The n+ drain 150 may be surrounded by amore lightly doped n-type shallow drain diffusion 152, as shown in FIG.1(b). Metallization 102 makes contact to the source diffusion 110 and(through p+ body contact diffusion 162) to body 160. The dopantconcentration of the P epi (or P well) layer 108 can be, for example, inthe range of 5e14 cm⁻³ to 5e17 cm⁻³. The doping of the P-substrate canbe in the range of 1e14 cm⁻³ to 5e19 cm⁻³.

FIG. 2(a) shows another embodiment with a different source pattern,which reduces resistance between the p+ body contact 162 and the body160. In this embodiment the n+ source diffusion 110 is laid out as aninterrupted strip rather than a continuous strip.

FIG. 2(b) shows another embodiment in which majority carrier flowthrough the channel is vertical rather than horizontal. Here the planargate 130 has been replaced by a trench gate 132. Note that the shape ofp-type body region 160 is somewhat different in this example.

An important parameter that controls the device switching power losses,is the total charge associated with charging or discharging thegate-drain capacitance Cgd. This charge is so called “Miller charge”Qgd. Therefore, it is important to reduce Qgd in order to reduce itstotal power losses. However, as the cell pitch reduces and the celldensity increases, the associated intrinsic capacitances of the device,such as the gate-to-drain capacitance (Cgd), also increase. As aconsequence, the switching power loss of the device increases.

The present application discloses several different device structureswhich improve the switching performance over prior art devices, and evenas compared to the teachings of the parent applications. FIGS. 3(a),3(b), and 3(c) show a lateral power device which is generally similar tothat shown in FIGS. 1(a)-1(c), except that it has an additionalconductive shield layer 311. The source metallization 102, n+ sourcediffusion 110, p-type body 160, p+ body contact 162, drift region 108,shallow drain 152, and deep drain 150 all function just as in the deviceof FIGS. 1(a)/1(b)/1(c). Thus the reduction in the parasitic Millercapacitance is achieved without harming device density or performance.

In this example the shield electrode 311 is coplanar with the gateelectrode 130, and is preferably fabricated in the same process step.

The conductive shield electrode 311 can be floating, or (morepreferably) is electrically shorted to the Source terminal. It can beformed by a polysilicon, polycide, or metal layer. As a result, the Gateelectrode 130 is more electrically shielded from the drain region thanin FIGS. 1(a)/1(b)/1(c), and the gate-to-drain capacitance Cgd islowered. Consequently the Miller charge Qgd is lowered, which reducesswitching power losses.

FIG. 4 shows another sample embodiment with the source metal layer 402is adapted to also provide some shielding for the gate electrode. Inthis example, a recess 407 has been etched into the thick dielectric405, to bring the outside edge of the source metallization down to alevel which is within a “line of sight” from some portion of the gateelectrode to at least some portion of the drain metallization 103.Source metal 402 thus has a shape which somewhat resembles a merger ofthe source metallization 102 of FIG. 1(a) etc. with the shield electrode311 of FIGS. 3(a), 3(b), and 3(c). Many elements in this Figure aresimilar to those described above, and those skilled in the art willreadily understand that descriptions of analogous elements in thepreceding Figures apply, with appropriate adjustments, to this Figuretoo.

FIGS. 5(a)-5(b) show another sample embodiment, with a source layoutwhich is slightly different from that of FIGS. 1(a)-1(c). Here too theshield electrode 511 can be floating, but more preferably iselectrically shorted to the Source terminal 102. The shield electrode511 can be formed from doped polysilicon or other conductive material.As a result, the Gate terminal 130 is electrically shielded from thedrain region, and the gate-to-drain charge Qgd is lowered. This reducesswitching losses. Many elements in these Figures are similar to thosedescribed above, and those skilled in the art will readily understandthat descriptions of analogous elements in the preceding Figures apply,with appropriate adjustments, to these Figures too.

FIG. 5(c) shows an alternative embodiment which uses a trench gate 132and a matching shield electrode 512. Note that the shield electrode 512,like the trench gate electrode 132, has a vertical cross section whichis much greater than its horizontal cross section. Since the shieldelectrode 512 is interposed between the gate electrode and the drain,Cgd is reduced. (Coupling capacitance will be affected by fringe fieldstoo, but interposition of an electrode into the line-of-sight does helpreduce coupling capacitance.)

Preferably the shield and gate electrodes are fabricated in the samekind of process. Optionally, it is contemplated that these twoelectrodes can be formed in the SAME process step, e.g. as sidewallfilaments on opposite faces of a trench. Many elements in this Figureare similar to those described above, and those skilled in the art willreadily understand that descriptions of analogous elements in thepreceding Figures apply, with appropriate adjustments, to this Figuretoo.

FIG. 6 shows another alternative embodiment which includes a shallowlayer 620, doped opposite to the body, at the surface of the driftregion 108. In this example, the device has an n-type surface region 620between the p-type body 160 and the N+ drain 150. Here too the presenceof the shield electrode 611 helps to reduce gate-drain capacitance, andhence Miller charge Qgd. Many elements in this Figure are similar tothose described above, and those skilled in the art will readilyunderstand that descriptions of analogous elements in the precedingFigures apply, with appropriate adjustments, to this Figure too.

FIGS. 7(a)-7(d) show other embodiments which include an n-type driftlayer and a p-type surface layer. The N-layer 708 in FIGS. 7(a) and 7(b)has uniform doping, e.g. as provided by an epitaxial layer. Manyelements in this Figure are similar to those described above, and thoseskilled in the art will readily understand that descriptions ofanalogous elements in the preceding Figures apply, with appropriateadjustments, to this Figure too.

FIGS. 7(c) and 7(d) show other embodiments, in which the N-layer 718 hasnon-uniform doping and can be formed by diffusion. Many elements in thisFigure are similar to those described above, and those skilled in theart will readily understand that descriptions of analogous elements inthe preceding Figures apply, with appropriate adjustments, to thisFigure too.

The embodiment in FIG. 7(d) includes an additional p-buried layer 719 inthe drift region 718. It should be noted that one or more p-buriedlayers 719 can be included. It should also be noted that in FIGS.7(a)-7(d) the presence of the shield electrode 711 helps to reducegate-drain capacitance, and hence Miller charge Qgd. Many elements inthis Figure are similar to those described above, and those skilled inthe art will readily understand that descriptions of analogous elementsin the preceding Figures apply, with appropriate adjustments, to thisFigure too.

FIG. 7(e) shows yet another alternative embodiment, which is somewhatsimilar to the device of FIG. 3(b), except for the use of a diffusedN-well 718 and a shallow trench 750. Trench 750 is filled withdielectric material, and preferably overlaps the gate electrode 130.This dielectric trench 750 helps to increase the physical distancebetween the gate electrode 130 and the majority carrier flows which passbeneath trench 750. This further reduces the gate-drain capacitance andimproves the device reliability. Many elements in this Figure aresimilar to those described above, and those skilled in the art willreadily understand that descriptions of analogous elements in thepreceding Figures apply, with appropriate adjustments, to this Figuretoo.

FIG. 8(a) shows another alternative embodiment which is generallysomewhat similar to those described above, but includes an n-type buriedlayer 820 within the drift region 108, between the p-type body 160 andthe N+ drain 150. This buried layer 820 will help to provide chargebalance under reverse-bias conditions. Here too the presence of theshield electrode 811 helps to reduce gate-drain capacitance, and henceMiller charge Qgd. Many elements in this Figure are similar to thosedescribed above, and those skilled in the art will readily understandthat descriptions of analogous elements in the preceding Figures apply,with appropriate adjustments, to this Figure too.

FIG. 8(b) shows another alternative embodiment which includes a shallowtrench 850 filled with dielectric material and located beneath the gateand the shield electrodes. The presence of the shallow trench 850overlapping the gate electrode results in a further reduction ingate-drain capacitance as well as improved device reliability due toreduction of surface electric field. Many elements in this Figure aresimilar to those described above, and those skilled in the art willreadily understand that descriptions of analogous elements in thepreceding Figures apply, with appropriate adjustments, to this Figuretoo.

FIGS. 9(a)-9(c) show other embodiments which are built in asemiconductor-on-insulator (SOI) structure. These examples use a planargate 130 and shield electrode 911. The gate and shield electrodes arepreferably formed at the same time, in a single thin-film layer. In thisexample the insulated substrate 905/906 is a silicon dioxide (905) onsilicon (906) structure, and the active layer 108 is preferably silicon.Here too the presence of the shield electrode 911 helps to reducegate-drain capacitance, and hence Miller charge Qgd. Many elements inthis Figure are similar to those described above, and those skilled inthe art will readily understand that descriptions of analogous elementsin the preceding Figures apply, with appropriate adjustments, to thisFigure too.

FIG. 10 shows another SOI embodiment. Unlike the device of FIGS.9(a)-(c), this device uses a trench gate 1012 with a matching shieldelectrode 1011. In either case, the gate and shield electrodes arepreferably formed by the same process. Here too the presence of theshield electrode helps to reduce gate-drain capacitance, and henceMiller charge Qgd. Many elements in this Figure are similar to thosedescribed above, and those skilled in the art will readily understandthat descriptions of analogous elements in the preceding Figures apply,with appropriate adjustments, to this Figure too.

FIG. 11(a) shows another device embodiment, which is generally somewhatsimilar to that of FIG. 3(a), except that the dielectric-filled trenches140 have been replaced by horizontally tapered dielectric-filledtrenches 1140. In the top view of FIG. 11(a), it can be seen that thetrench width is narrower at the drain side. For a given areal density offixed charge along the sidewalls of the trench, this means that thecharge balance ratio varies along the length of the drift region, withless charge balancing at the drain end of the trenches 1140. Here toothe presence of the shield electrode helps to reduce gate-draincapacitance, and hence Miller charge Qgd. Many elements in this Figureare similar to those described above, and those skilled in the art willreadily understand that descriptions of analogous elements in thepreceding Figures apply, with appropriate adjustments, to this Figuretoo.

FIG. 11(b) is a top view of another lateral device structure, which isgenerally similar to that of FIG. 11(a), except that the trenches 1140have been replaced by trenches 1141 which have an opposite direction oftaper: trenches 1141 are wider at their drain end. For a given arealdensity of fixed charge along the sidewalls of the trench, this meansthat the charge balance ratio varies along the length of the driftregion, with less charge balancing at the source end of the trenches1140. Here too the presence of the shield electrode helps to reducegate-drain capacitance, and hence Miller charge Qgd. Many elements inthis Figure are similar to those described above, and those skilled inthe art will readily understand that descriptions of analogous elementsin the preceding Figures apply, with appropriate adjustments, to thisFigure too.

Additionally, further improvement in Qgd can be obtained by reducing thefringing capacitance in the transition region between the gate and theShield Layer. This is realized by controlling the oxide thickness in thetransition zone.

The doping levels needed to achieve high breakdown and low-resistanceare governed by the well known charge balance condition. The specificelectrical characteristics of devices fabricated using the methodsdescribed in this disclosure depend on a number of factors including thethickness of the layers, their doping levels, the materials being used,the geometry of the layout, etc. One of ordinary skill in the art willrealize that simulation, experimentation, or a combination thereof canbe used to determine the design parameters needed to operate asintended.

According to some, but not necessarily all disclosed inventiveembodiments, there is provided: A semiconductor device comprising: afirst-conductivity-type source region; a second-conductivity-type bodyregion interposed between said source region and a semiconductor driftregion; a gate electrode which is capacitively coupled to controllablyinvert a portion of said body region, to controllably form therein achannel which connects said source region to said drift region; whereinsaid drift region is laterally interposed between said body region and afirst-conductivity-type drain region; permanent charge, embedded in atleast one insulating region which vertically adjoins said drift region,which has a polarity which tends to deplete a layer of said drift regionin proximity to said insulating region; and a shield electrode, which isat least partly interposed between said gate electrode and said drain toreduce capacitive coupling between said gate and said drain.

According to some, but not necessarily all disclosed inventiveembodiments, there is provided: A semiconductor device comprising: afirst-conductivity-type source region; a second-conductivity-type bodyregion interposed between said source region and a semiconductor driftregion of said second conductivity type; a gate electrode which iscapacitively coupled to controllably invert a portion of said bodyregion, to controllably form therein a channel which connects saidsource region to said drift region; wherein said semiconductor driftregion is laterally interposed between said body region and afirst-conductivity-type drain region; immobile positive netelectrostatic charge, capacitively coupled to said drift region to expelholes from a continuous portion of said drift region which extends fromsaid channel to a drain diffusion; and a shield electrode, which is atleast partly interposed between said gate electrode and said drain toreduce capacitive coupling between said gate and said drain.

According to some, but not necessarily all disclosed inventiveembodiments, there is provided: A method of operating a semiconductordevice, comprising: in the ON state, applying a voltage to an insulatedgate to thereby invert a portion of a second-conductivity-type bodyregion interposed between a first-conductivity-type region and asecond-conductivity-type drift region, and thereby allow passage ofmajority carriers from said source region through said channel, andthrough a portion of said drift region which has been inverted byimmobile permanent net electrostatic charge, to afirst-conductivity-type drain region; and in the OFF state, balancingthe space charge of depleted portions of said drift region with said netelectrostatic charge; and reducing capacitive coupling by a shieldelectrode which is interposed between said gate electrode and saiddrain.

According to some, but not necessarily all disclosed inventiveembodiments, there is provided: Lateral power devices where immobileelectrostatic charge is emplaced in dielectric material adjoining thedrift region. A shield gate is interposed between the gate electrode andthe drain, to reduce the Miller charge. In some embodiments the gateelectrode is a trench gate, and in such cases the shield electrode toois preferably vertically extended.

According to some, but not necessarily all disclosed inventiveembodiments, there is provided: A semiconductor device comprising: afirst-conductivity-type source region; a second-conductivity-type bodyregion interposed between said source region and a semiconductor driftregion of said second conductivity type; a gate electrode which iscapacitively coupled to controllably invert a portion of said bodyregion, to controllably form therein a channel which connects saidsource region to said drift region; wherein said semiconductor driftregion is laterally interposed between said body region and afirst-conductivity-type drain region; and an insulated trench whichlaterally adjoins said drift region, and which contains immobile netelectrostatic charge which is capacitively coupled to said drift region,and which is laterally tapered.

According to some, but not necessarily all disclosed inventiveembodiments, there is provided: A semiconductor device comprising: afirst-conductivity-type source region; a second-conductivity-type bodyregion interposed between said source region and a semiconductor driftregion; and a gate electrode which is capacitively coupled tocontrollably invert a portion of said body region, to controllably formtherein a channel which connects said source region to said driftregion; wherein said drift region is laterally interposed between saidbody region and a first-conductivity-type drain region; permanentcharge, which is embedded in at least one insulating region whichadjoins said drift region, and which has a polarity which tends todeplete a layer of said drift region in proximity to said insulatingregion; a shield electrode, which is at least partly interposed betweensaid gate electrode and said drain to reduce capacitive coupling betweensaid gate and said drain; and a dielectric-filled trench within saiddrift region, which lies at least partly beneath said gate electrode.

Modifications and Variations

As will be recognized by those skilled in the art, the innovativeconcepts described in the present application can be modified and variedover a tremendous range of applications, and accordingly the scope ofpatented subject matter is not limited by any of the specific exemplaryteachings given. It is intended to embrace all such alternatives,modifications and variations that fall within the spirit and broad scopeof the appended claims.

While the figures shown in this disclosure are qualitatively correct,the geometries used in practice may differ and should not be considereda limitation in anyway. It is understood by those of ordinary skill inthe art that the actual cell layout will vary depending on the specificsof the implementation and any depictions illustrated herein should notbe considered a limitation in any way.

While only n-channel MOSFETs are shown here, p-channel MOSFETs arerealizable with this invention simply by changing the polarity of thepermanent charge and swapping n-type and p-type regions in any of thefigures. This is well known by those of ordinary skill in the art.

This invention is also applicable to the case of correspondingstructures that use negative permanent charge and replacing the P regionbetween the p-body and drain regions with an N-region. Permanentnegative charge can be created for example by using different dielectriclayers such as silicon dioxide and Aluminum Oxide.

It is understood that the permanent charge can be in the dielectric(oxide), at the interface between the silicon and oxide, inside thesilicon layer or a combination of all these cases.

It is also understood that numerous combinations of the aboveembodiments can be realized.

It is understood by those of ordinary skill in the art that othervariations to the above embodiments can be realized using other knowntermination techniques.

None of the description in the present application should be read asimplying that any particular element, step, or function is an essentialelement which must be included in the claim scope: THE SCOPE OF PATENTEDSUBJECT MATTER IS DEFINED ONLY BY THE ALLOWED CLAIMS. Moreover, none ofthese claims are intended to invoke paragraph six of 35 USC section 112unless the exact words “means for” are followed by a participle.

The claims as filed are intended to be as comprehensive as possible, andNO subject matter is intentionally relinquished, dedicated, orabandoned.

What is claimed is:
 1. A method of operating a semiconductor device,comprising: in the ON state, applying a voltage to an insulated gate tothereby invert a portion of a second-conductivity-type body regioninterposed between a first-conductivity-type source region and asecond-conductivity-type drift region to thereby form a channel, andthereby allow passage of majority carriers from said source regionthrough said channel, and through a portion of said drift region whichhas been inverted by immobile permanent net electrostatic charge, to afirst-conductivity-type drain region; and in the OFF state, at leastpartially balancing the space charge of depleted portions of said driftregion with said immobile permanent net electrostatic charge; andreducing capacitive coupling by a shield electrode which is interposedbetween said gate electrode and said drain.
 2. The method of claim 1,wherein said drift region is a well.
 3. The method of claim 1, wherein,in the ON state, said channel and said drift region both carry currentflow in a predominantly horizontal direction.
 4. The method of claim 1,wherein, in the ON state, said channel carries a flow of majoritycarriers in a plane which is predominantly normal to the plane of flowof majority carriers in said drift region.
 5. The method of claim 1,wherein, in the ON state, said channel carries a flow of majoritycarriers in a plane which is predominantly normal to the surface of asemiconductor die, and said drift region carries a flow of majoritycarriers which is predominantly parallel to said surface.
 6. The methodof claim 1, wherein said gate electrode and said shield electrode bothlie in the same plane of metallization.
 7. The method of claim 1,wherein said shield electrode is connected to a fixed potential, butsaid gate electrode is not.